The present invention relates to frequency control apparatus and more particularly to a watchdog or safety timer that detects abnormal operation of a circuit incorporating a clock oscillator.
Many electronic devices incorporate internal clock circuits as standard time bases for maintaining particular timing relationships or for synchronization with other devices. To assure proper operation or to detect any abnormality in timing, a form of watchdog timer is sometimes used. Conventional watchdog timers include mono multivibrators that use a clock signal as a trigger signal and timers that use the clock signal as a reset. Due to the operational features of these devices such circuits are capable of determining when the clock frequency has dropped below a preselected "normal" level. However, these circuits cannot detect when the clock frequency has increased beyond a desirable operating level. The use of condensers as part of a discharge/recharge cycle and detection process fails at higher frequencies because the period of operation becomes too short to allow sufficient changes in voltage level to be detected. These and other problems have prevented adequate safety and reliability in watchdog timers.